Pollution is being removed more slowly from the atmosphere in the northern hemisphere than previously assumed. An international research team that includes researchers from Wageningen University established that the atmospheric cleaning agent hydroxyl (OH) is found in the same concentrations in the southern and the northern hemisphere. Until now, scientists had assumed that there would be more OH in the atmosphere in the more polluted northern hemisphere as the cleaning agent is produced where there is pollution. These results will be published in Nature on 11 September.

The atmosphere is much more polluted in the northern hemisphere than in the southern hemisphere. This is because, while the southern hemisphere mainly consists of oceans, the northern hemisphere includes the large continents of Asia, Europe and North America and their industry and traffic. Exhaust fumes from cars and factories as well as greenhouse gases such as methane and pollutants such as nitrogen oxides enter the atmosphere. Chemical processes involving nitrogen oxides that take place in the atmosphere stimulate the formation of the cleaning agent OH (the ‘hydroxyl-radical’). As there is more pollution in the northern hemisphere, there should therefore also be more OH in the northern hemisphere; according to model calculations about 25 % more.

Atmospheric cleaning agent

The hydroxyl-radical is essential for removing pollution from the atmosphere. The molecule exists for just a few seconds at the most, but in that time the OH radical reacts with and breaks down pollutants in the air. However, the short lifespan makes it difficult to measure exactly how much of the OH cleaning agent is present in the atmosphere.

Smart monitoring

Because of this, the researchers measured the amount of OH indirectly by monitoring the amount of methyl chloroform. This is a man-made chemical compound that has been banned under the Montreal Protocol – a global agreement that was designed to stop the emission of chemical gases that deplete the ozone layer. However, man-made methyl chloroform is still found in the atmosphere, although the gas is gradually and almost exclusively broken down by OH. OH concentrations can therefore be determined by monitoring methyl chloroform levels in the atmosphere: a low level of methyl chloroform indicates a high concentration of the OH-radical. Measurements from monitoring stations in Ireland and Tasmania in the southern atmosphere show that OH concentrations in both hemispheres must be about the same.

The methyl chloroform data was obtained from two atmospheric monitoring networks (the Advanced Global Atmospheric Gases Experiment (AGAGE) and the National Oceanic and Atmospheric Administration (NOAA)) and from data from an aeroplane that flew from pole to pole (High-performance Instrumented Airborne Platform for Environmental Research Pole-to-Pole Observations (HIPPO)). The researchers combined the monitoring data with a computer model to determine the OH concentrations in both the northern and the southern hemisphere. They were surprised to discover that the OH concentration was about the same in both hemispheres. ‘This shows that our understanding of the atmospheric chemistry of the hydroxyl-radical is still incomplete’, concluded Maarten Krol, researcher at Wageningen University’s Meteorology and Air Quality Group.